1. Field of the Invention
The invention relates to a method for data processing in an optical network, to an optical network component and to a communication system comprising such optical network component.
A passive optical network (PON) is a promising approach regarding fiber-to-the-home (FTTH), fiber-to-the-business (FTTB) and fiber-to-the-curb (FTTC) scenarios, in particular as it overcomes the economic limitations of traditional point-to-point solutions.
The PON has been standardized for FTTH solutions and it is currently being deployed by network service providers world-wide. Conventional PONs distribute downstream traffic from the optical line terminal (OLT) to optical network units (ONUS) in a broadcast manner while the ONUS send upstream data packets multiplexed in time to the OLT. Hence, communication among the ONUS needs to be conveyed through the OLT involving electronic processing such as buffering and/or scheduling, which results in latency and degrades the throughput of the network.
Upstream signals can be combined using a multiple access protocol, i.e. invariably time division multiple access (TDMA). The OLTs “range” the ONUS to provide time slot assignments for upstream communication. Hence, the data rate is distributed among several subscribers, whereas the single ONU needs to be capable of handling data rates that are significantly higher than the average data rate utilized by such ONU.
Future traffic is assessed to amount to ca. 1 Gb/s per subscriber regarding the majority of subscribers. A small portion of subscribers (compared to the number of total subscribers) will be provided with a data rate in the range of ca. 10 Gb/s. It would be very cost-ineffective and a plain waste of resources to equip all subscribers with ONUs that are capable of processing high data rates of, e.g., 10 Gb/s, wherein only a small portion of subscribers will actually receive such high data rates.
2. Brief Summary of the Invention
The problem to be solved is to overcome the disadvantages stated above and in particular to provide a flexible and cost effective optical network, e.g., a passive optical network or an optical access network.
This problem is solved according to the features of the independent claims. Further embodiments result from the depending claims.
In order to overcome this problem, a method for data processing in an optical network is provided comprising the steps                (a) providing at least one main wavelength;        (b) processing a subcarrier modulation for said at least one main wavelength, wherein a portion of the subcarrier modulated signal is suppressed.        
Suppressing a portion, e.g., a side band of the subcarrier modulated signal allows to efficiently utilize the optical power and to economically supply a huge number of subscribers.
It is noted that the approach provides for coherent data transmission in a point-to-point or point-to-multipoint optical network.
The optical network may comprise various structures, e.g., a ring or a bus topology or mixtures thereof.
According to an embodiment, the at least one main wavelength is an optical carrier that is in particular provided by at least one optical comb generator and/or by several light sources, in particular by several lasers.
In particular, several optical comb generators can be used to provide interleaving main wavelengths, wherein each such wavelength can be further utilized for subcarrier modulation.
In another embodiment, the subcarrier modulation comprises an optical and/or an electrical modulation.
Hence, subcarrier modulation can be done either optically and/or electronically. For example, a set of subcarriers can be generated by optical modulation of a main wavelength.
It is noted that various types of modulation may be applicable, e.g., OOK, QAM, QPSK, etc.
Integrated modulators can be used in particular deployed as an array structure to combine the user data with the subcarriers.
In a further embodiment, in step (b) the subcarrier modulation suppressing a portion of the modulated signal comprises substantially a single sideband modulation.
Hence, via single sideband modulation a portion of said subcarrier modulated signal is suppressed.
In a next embodiment, in step (b) an optical filtering is performed in order to suppress a portion of the subcarrier modulated signal, in particular in order to suppress substantially at least a portion of the side band.
Thus, as an alternative, filtering may apply in order to suppress said portion of the subcarrier modulated signal. Such can be achieved, e.g., by means of an optical filter.
As another alternative, electronic filtering may apply. Advantageously, optical and electronic filtering may be combined in order to suppress said portion of the subcarrier signal.
It is also an embodiment that the main wavelength is suppressed, in particular by means of destructive interference.
This can be achieved by feeding the main wavelength to a modulator and to a 180° phase shifter and combining the outputs of such modulator and the phase shifter thereinafter. This allows to obtain the modulated signal without the main wavelength.
In an embodiment, subcarriers for said subcarrier modulation are provided optically and/or electronically in particular based on each said at least one main wavelength.
Preferably, an array of modulators can be utilized in order to process the at least one main wavelength and to provide the modulated signal(s).
Advantageously, each subcarrier may carry user data and is supplied to at least one subscriber.
Pursuant to another embodiment, the at least one subcarrier or a set of subcarriers is assigned to one subscriber or to several subscribers.
Hence, one subscriber may sign up for a higher bandwidth thereby being provided with several subcarriers to convey user data. As an alternative, the spacing between subcarriers may vary in order to allow higher bandwidth to be conveyed to the subscriber.
This concept advantageously applies in downstream direction as well as in upstream direction.
Preferably, in downstream direction, an optical line terminal (OLT) supplies several optical network units (ONUs), wherein in upstream direction traffic is conveyed from the ONUS towards the OLT.
It is possible to convey upstream traffic from several ONUS via overlapping or same wavelengths, but modulated at each ONU on different disjoint sub-carrier frequencies.
It is noted that one light source provided, e.g., by the optical comb generator or by an individual laser, is utilized as optical local oscillator for a set of ONUS.
According to an embodiment, different subcarriers are utilized for upstream and downstream data.
In particular, different subcarriers or sets of subcarriers can be assigned to different receiving units and/or to different transmitting units (at the OLT as well as at the ONU). This advantageously allows using the subcarriers in a flexible way to meet the subscriber's demand for bandwidth and to provide an effective and non-expensive optical network.
It is in particular of advantage to use disjoint subcarriers or disjoint sets of subcarriers in upstream and downstream direction.
It is noted that a transmitter at the ONU may be realized as a reflective transmitter.
According to another embodiment, the at least one main wavelength is provided by a local oscillator. The main wavelength provided by the local oscillator can further be utilized for receiving and transmitting purposes.
Hence, in particular at the ONU, the local oscillator signal can be used for receiving as well as transmitting purposes.
The problem stated supra is solved by an optical network component                (a) arranged to provide at least one main wavelength;        (b) arranged to process a subcarrier modulation for said at least one main wavelength, wherein a portion of the subcarrier modulated signal is suppressed.        
As an embodiment, the at least one main wavelength is an optical carrier that is in particular provided by at least one optical comb generator and/or by several light sources, in particular by several lasers.
The problem stated above is also solved by an optical network component comprising a and/or being associated with a processor unit and/or a hard-wired circuit and/or a logic device and/or optical components that is arranged such that the method as described herein.
According to an embodiment, the optical network component is a or is associated with an optical line terminal and/or an optical network unit.
The optical network component may be or it may be associated with a coherent receiver comprising                means for electrical down-conversion to baseband of the signal received; and        means for detecting data symbols in particular via analog-to-digital conversion followed by digital processing.        
A coherent receiver may also be arranged to separate subcarriers by electrical filtering and to detect data symbols via analog-to-digital conversion followed by digital processing.
The problem stated supra is further solved by a communication system, in particular a PON, comprising the optical network component as described herein.
It is an embodiment that at least one optical network unit is supplied by a signal from an local oscillator, said local oscillator being deployed external or internal to the optical network unit.
It is noted that said optical network may comprise or be an optical access network (OAN) that is in particular based on a tree topology and comprises at least one passive optical splitter and utilizes the transmission and/or receiving concept as described herein. Said OAN may comprise (coarse) WDM multiplexer, passive optical splitters, (bi-directional) optical amplifiers.
Embodiments of the invention are shown and illustrated in the following figures: